revlog: made C Capsule an array of function pointers

revlog: made C Capsule an array of function pointers

Although it's perfectly valid to put a function pointer in
a capsule, as we've been doing since the start of rust/hg-cpython,
an array of function pointers has several advantages:

- it can hold several functions. That's our main motivation here.
  We plan to expose index_length() and index_node(), which will
  be needed for a Rust implementation of nodemap.
- it could also have data
- (probably minor in the case of Mercurial) proper support for
  architectures for which data and code pointers don't have the
  same size.

Differential Revision: https://phab.mercurial-scm.org/D7543

--- a/mercurial/cext/revlog.c	Sun Dec 22 23:09:37 2019 -0500
+++ b/mercurial/cext/revlog.c	Wed Nov 27 17:59:58 2019 +0100
@@ -37,6 +37,10 @@
 	int children[16];
 } nodetreenode;
 
+typedef struct {
+	int (*index_parents)(PyObject *, int, int *);
+} Revlog_CAPI;
+
 /*
  * A base-16 trie for fast node->rev mapping.
  *
@@ -3032,6 +3036,10 @@
 };
 #endif /* WITH_RUST */
 
+static Revlog_CAPI CAPI = {
+    HgRevlogIndex_GetParents,
+};
+
 void revlog_module_init(PyObject *mod)
 {
 	PyObject *caps = NULL;
@@ -3055,11 +3063,9 @@
 	if (nullentry)
 		PyObject_GC_UnTrack(nullentry);
 
-	caps = PyCapsule_New(HgRevlogIndex_GetParents,
-	                     "mercurial.cext.parsers.index_get_parents_CAPI",
-	                     NULL);
+	caps = PyCapsule_New(&CAPI, "mercurial.cext.parsers.revlog_CAPI", NULL);
 	if (caps != NULL)
-		PyModule_AddObject(mod, "index_get_parents_CAPI", caps);
+		PyModule_AddObject(mod, "revlog_CAPI", caps);
 
 #ifdef WITH_RUST
 	rustlazyancestorsType.tp_new = PyType_GenericNew;

--- a/rust/hg-cpython/src/cindex.rs	Sun Dec 22 23:09:37 2019 -0500
+++ b/rust/hg-cpython/src/cindex.rs	Wed Nov 27 17:59:58 2019 +0100
@@ -14,15 +14,18 @@
 use hg::{Graph, GraphError, Revision, WORKING_DIRECTORY_REVISION};
 use libc::c_int;
 
-py_capsule_fn!(
-    from mercurial.cext.parsers import index_get_parents_CAPI
-        as get_parents_capi
-        signature (
-            index: *mut RawPyObject,
-            rev: c_int,
-            ps: *mut [c_int; 2],
-        ) -> c_int
-);
+#[repr(C)]
+pub struct Revlog_CAPI {
+    index_parents: unsafe extern "C" fn(
+        index: *mut revlog_capi::RawPyObject,
+        rev: c_int,
+        ps: *mut [c_int; 2],
+    ) -> c_int,
+}
+
+py_capsule!(
+    from mercurial.cext.parsers import revlog_CAPI
+        as revlog_capi for Revlog_CAPI);
 
 /// A `Graph` backed up by objects and functions from revlog.c
 ///
@@ -58,14 +61,14 @@
 /// mechanisms in other contexts.
 pub struct Index {
     index: PyObject,
-    parents: get_parents_capi::CapsuleFn,
+    capi: &'static Revlog_CAPI,
 }
 
 impl Index {
     pub fn new(py: Python, index: PyObject) -> PyResult<Self> {
         Ok(Index {
             index: index,
-            parents: get_parents_capi::retrieve(py)?,
+            capi: unsafe { revlog_capi::retrieve(py)? },
         })
     }
 }
@@ -75,7 +78,7 @@
         let guard = Python::acquire_gil();
         Index {
             index: self.index.clone_ref(guard.python()),
-            parents: self.parents.clone(),
+            capi: self.capi,
         }
     }
 }
@@ -88,7 +91,7 @@
         }
         let mut res: [c_int; 2] = [0; 2];
         let code = unsafe {
-            (self.parents)(
+            (self.capi.index_parents)(
                 self.index.as_ptr(),
                 rev as c_int,
                 &mut res as *mut [c_int; 2],